Wow!, that is quite a bit of disk consumption. Do you have any idea where most of it went (pictures, music, movies?). If you don't know, there are a few tools out there which give a human-readable graphic display of where all your drive space is going. SpaceMonger is probably one of the more popular ones. There is also TreeSize

You have 2 choices,
1) move the big stuff off of there -- but if you don't know where the space is going, then that will be a challenge. If you decide to go this route, create some folders on D:\ where the various file types will go.

2) resize the C: partition -- Easeus has a free-for-home partition re-sizer which is very easy to use. http://www.partition-tool.com/personal.htm . It will take space from D: (as much as you want) and put it on C: .

Also, if this data is important to you back it up to an external hard drive.